The invention relates to a head-mounted display provided with an image display device and an optical system comprising a semi-transmissive mirror, a lens element, a polarizing element and a xcex/4 plate for combining light beams so as to project an image to be formed by the image display device on a user""s retina.
A device of the type described above is known from European patent application EP 0 718 645. In a head-mounted display described in this document, an image is formed with a liquid crystalline image display panel, which image is projected by means of the optical system on the user""s retina via the exit pupil of the head-mounted display. During use, the exit pupil coincides with an eye pupil of a user of the head-mounted display so that an image is formed on the user""s retina. The head-mounted display may be worn by a user so as to display, for example, TV or video images or images which are associated with portable information and communication systems, computer games or computer simulations.
In the known head-mounted display, the optical system, viewed from the image display device, consecutively comprises a first xcex/4 plate, a semi-transmissive mirror, a lens element and a polarization-dependent mirror. By partial reflection it is achieved that radiation of the image to be formed effectively passes the lens element three times. Furthermore, the first surface of the polarization-dependent mirror may be curved so as to reduce optical imaging errors. To obtain the polarization-dependent mirror, a polarization-dependent layer is then provided on the curved surface.
A drawback of the known head-mounted display is that the viewing angle at which a user observes the image is limited.
It is an object of the invention to provide a head-mounted display in which the viewing angle at which a user observes the image is increased. This object is achieved by the device according to the invention, as defined in claim 1. The advantage of this configuration is that the magnification is now mainly determined by the shape of the semi-transmissive mirror. Furthermore, a curved mirror may yield a higher magnification factor than is possible with the known combination of a flat mirror and a lens element. Moreover, the semi-transmissive mirror may have a semi-transparent layer which can be provided in a conventional and simple manner. A further advantage is that the polarization-dependent layer can be easily provided on a flat plate for the manufacture of the polarization-dependent mirror. Further advantageous embodiments are defined in the dependent claims.
A particular embodiment of the head-mounted display according to the invention is characterized in that an optical axis of the first xcex/4 plate is transverse to an optical axis of the second xcex/4 plate. This results in achromatic transmission suppression so that the contrast of the image is enhanced. This measure is known per se from the quoted European patent application EP 0 803 756.
A further embodiment of the head-mounted display according to the invention is characterized in that the polarization-dependent mirror is transmissive to a first direction of polarization of a linearly polarized light beam and is reflective to a second direction of polarization of the linearly polarized light beam, which direction of polarization is perpendicular to the first direction of polarization. An example of such a polarization-dependent mirror is a reflecting polarizing element such as, for example, Double Brightness Enhancement Foil as supplied by 3M.
A further embodiment of the head-mounted display according to the invention is characterized in that the lens element comprises a plano-concave lens element. For example, the concave surface of such a plano-concave lens is directed towards the second xcex/4 plate. The concave surface of the semi-transmissive mirror makes a magnified virtual image of the image display device. Moreover, the use of a plano-concave lens results in a saving of the total weight of the head-mounted display, which is more comfortable for the user.
A further embodiment of the head-mounted display according to the invention is characterized in that the head-mounted display comprises an integrated optical element comprising the first xcex/4 plate, the lens element and the semi-transmissive mirror. Integration of optical components in an optical element simplifies the assembly of the head-mounted display during production. The second xcex/4 plate and the polarization-dependent mirror may also be integrated in a single optical element.
A further embodiment of the head-mounted display according to the invention is characterized in that the head-mounted display comprises at least an optical element which has an aspherical surface. The use of optical elements having aspherical surfaces reduces imaging errors such as coma, astigmatism and curvature of the field.
A further embodiment of the head-mounted display according to the invention is characterized in that the first and the second xcex/4 plate are achromatic. The color errors in the images are reduced with such xcex/4 plates.
A further embodiment of the head-mounted display according to the invention is characterized in that the second xcex/4 plate and the polarization-dependent mirror have curved surfaces. Optionally, either the resolution may be enhanced or the magnification factor may be increased.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.